Connected: An Internet Encyclopedia
RFC 1510 - 6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5des)

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### 6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5des)

The RSA-MD5-DES checksum calculates a keyed collisionproof checksum by prepending an 8 octet confounder before the text, applying the RSA MD5 checksum algorithm, and encrypting the confounder and the checksum using DES in cipher-block-chaining (CBC) mode using a variant of the key, where the variant is computed by eXclusive-ORing the key with the constant F0F0F0F0F0F0F0F0. The initialization vector should be zero. The resulting checksum is 24 octets long (8 octets of which are redundant). This checksum is tamper-proof and believed to be collision-proof.

The DES specifications identify some "weak keys"; those keys shall not be used for encrypting RSA-MD5 checksums for use in Kerberos.

The format for the checksum is described in the following diagram:

          +--+--+--+--+--+--+--+--
          |  des-cbc(confounder
          +--+--+--+--+--+--+--+--

                         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                             rsa-md5(confounder+msg),key=var(key),iv=0)  |
                         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

The format cannot be described in ASN.1, but for those who prefer an ASN.1-like notation:

       rsa-md5-des-checksum ::=   ENCRYPTED       UNTAGGED SEQUENCE {
                                  confounder[0]   UNTAGGED OCTET STRING(8),
                                  check[1]        UNTAGGED OCTET STRING(16)
       }

Next: 6.4.6. DES cipher-block chained checksum (des-mac)

Connected: An Internet Encyclopedia
RFC 1510 - 6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5des)